The Union of Brest was the 1595-1596 decision of a number of Orthodox bishops in the region of what is modern Ukraine, Poland and Belarus ("Rus'") to depart from the Orthodox Church and place themselves under the Pope of Rome in order to avoid being ruled by the newly established Patriarch of Moscow. Thus was formed the Unia, from whence derives the term Uniate.

At the time, the church in the area included most Ukrainians and Belarusians, under the rule of the Polish-Lithuanian Commonwealth. The hierarchs of the Kievan church gathered in synod in the city of Brest to compose the union's 33 articles, which were then accepted by the Roman Catholic pope. At first widely successful, within several decades it lost much of its initial support. In Austrian Galicia, however, the church fared well and remains strong to this day, most notably in the Ukrainian Greek Catholic Church.

The union was strongly supported by the king of Poland and grand duke of Lithuania, Sigismund III Vasa, but opposed by some bishops and prominent nobles of Rus' and perhaps most importantly by the nascent Cossack movement for Ukrainian self-rule. The result was "Rus' fighting against Rus'" and the splitting of many traditionally Orthodox Christian people from their ancestral Church.

Background

A large area in the southwest of the Rusyn Empire became absorbed by Lithuania and Poland after the destruction of Kievan power by the Tartars. This southwestern part of Rus' was known as Little Rus' (which in Latin became Ruthenia); this is the territory that is present day Ukraine. In 1386, the kingdoms of Poland and Lithuania were united under a single ruler. The monarch of the united realm was Roman Catholic, and a substantial minority of the population were Orthodox. These Orthodox were in a difficult situation because the Patriarch of Constantinople, to whose jurisdiction they belonged, could exercise no control in Poland, as the former Byzantine capital had fallen to the Muslim Turks. The bishops were appointed not by the Church but by the Roman Catholic king of Poland.

The authorities in Poland always tried to make the Orthodox submit to the pope to reunify Christianity. With the arrival of the Jesuits in 1564, pressure on the Orthodox increased. The state of the church in the area was poor; clergy were uneducated and the bishops were without the funds they needed to properly run the church. Many priests were ordained without basic training and new rites were developing that were neither Latin nor Greek in their character. Constantinople was under Muslim rule and Moscow had recently been elevated to the status of patriarchate. The bishops of the Rus' were stuck between a population converting to Roman Catholicism on the West and a rising Muscovite force in the East.

At the synod in Brest six out of eight Orthodox bishops—including the Metropolitan of Kiev, Michael Ragoza—supported the union, but the remaining three bishops from the extreme west of Ukraine and eastern Poland (Lviv, Lutsk, and Przemyśl) would not join the union until later (1700, 1702, and 1693 respectively). The Cossack forces of Ukraine felt the union was a betrayal to the Polish rulers and united with the Russian Empire to fight against Poland and all who supported the empire, including the Greek-Catholics.

In 1620 Patriarch Theophanes III of Jerusalem arrived in Kiev and consecrated an Orthodox hierarchy, including Job (Boretsky) as Metropolitan of Kiev, for the church and thus there emerged a situation of both Orthodox and Eastern Catholic bishops coexisting in the same territory in Ukraine from that point onwards.